The present invention relates to disk drives and, more particularly, to a control circuit in a system for positioning a read/write head of a hard disk drive.
A hard disk drive comprises typically a plurality of rotatable disks with generally concentric data tracks containing digital information.
A head is associated with each disk for reading and writing data onto the various tracks when the disk is rotated and an actuator is connected to the head for moving the head to a desired track and maintaining it over the track centerline during read/write operations.
The actuator comprises typically a voice coil motor, or VCM, having a coil movable through the magnetic field of a permanent magnetic stator. The application of a current to the VCM causes the coil, and thus the head connected thereto, to move radially. The acceleration of the coil is proportional to the applied current, so that ideally no current flows through the coil if the head is stationary over a desired track.
FIG. 1 shows a known circuit for driving a VCM 10 which controls the position of a disk head. The VCM 10 is represented by a series connection of an inductor Lm (motor coil inductance) and a resistor Rm (motor coil ohmic resistance) and is powered by a power bridge amplifier, represented by two operational amplifiers 11 and 12 having gains G and xe2x88x92G, respectively. The amplifier 11 has its non-inverting input connected to the inverting input of the amplifier 12. The inverting input of amplifier 11 and the non-inverting input of amplifier 12 are connected to a source of a reference voltage VREF (not shown). A sensing resistor Rs is connected in series with the VCM 10 for providing a voltage signal to the input of a sense amplifier 13 having a gain Gs.
A control voltage is applied to the bridge amplifier through an error amplifier 14 having its non-inverting input coupled to the reference voltage VREF and its inverting input coupled to a source (not shown) of an analog voltage Vin through a series input resistor Ri. The output and the inverting input of error amplifier 14 are connected to one another through a series connection of a gain control resistor Rc and a frequency compensation capacitor Cc.
The output of sense amplifier 13 is connected to the inverting input of error amplifier 14 through a feedback resistor Rf.
In operation, a signal comprising information on the position which is desired for the disk head is applied as a voltage Vin to the error amplifier 14. The power bridge amplifier 11, 12 is driven by the output voltage from the error amplifier 14 and provides the VCM 10 with a current Im. The current Im is controlled through the feedback loop comprising the sense resistor Rs, the sense amplifier 13, the feedback resistor Rf and the error amplifier 14.
The transfer function of the circuit of FIG. 1 can be calculated as       Im    Vin    =            -              xe2x80x83            ⁢              Rf        Ri              *          1              Rs        *        Gs            
In a practical implementation of the circuit of FIG. 1 as a portion of an integrated circuit four pads are required for connection of external components, besides the pads for connection of the VCM 10 and the sensing resistor Rs. More particularly, one pad 15 is required to sense the reference voltage VREF for calibration and test purposes and three pads 16, 17 and 18 are required for connection of resistors Ri, Rc, Rf and capacitor Cc. In fact, resistors Ri, Rc, Rf should be high precision resistors and capacitor Cc should be a relatively large component; therefore they cannot be integrated with the rest of the circuit.
The present trend in the design of VCM control systems is towards the use of a digital signal instead of an analog signal for providing the head position information. To use the control circuit of FIG. 1 with a digital signal it is necessary to convert the digital signal into an analog signal. FIG. 2 shows the circuit of FIG. 1 with an input section comprising a digital-to-analog converter (DAC) 20 and a buffer 21. The DAC 20 has a digital input provided by a microprocessor 19 on a serial port and an analog output. The buffer 21 is necessary for coupling the high-impedance DAC output to the relatively low-impedance input of the control circuit.
The arrangement shown in FIG. 2 requires an additional pad 22 and an additional component (the buffer 21). The additional pad occupies area on the chip and implies that the control signal goes out from the chip and is subject therefore to external disturbances and the buffer occupies extra area and brings about offset problems which render the overall design more complex.
An embodiment of this invention provides a control circuit for a hard disk drive with digital input which does not require additional pads or additional components.
The control circuit includes drive means for generating a control current for use by the voice coil motor, sensor means for sensing the motor control current and amplifier means having an inverting input connected to a reference voltage source through resistance means and to an output of the sensor means, a non-inverting input connected to the source of digital position signals through digital-to-analog converting means and an output connected to an input of said drive means.